This is a far better question than the previous version: simple, clear, direct and unambiguous. It would be even better if you could share what research you have done yourself (even if that research was ultimately fruitless).
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Andrew Leach♦Nov 28 '13 at 9:12

Okay I'll bite, here's a verb: love. Please do ask your colleague to come up with 13 inflectional forms for it. We'll wait. While we are waiting, perhaps you could explain what use the number is in the first place. What does it mean or why does it matter if English has 4 inflectional forms, or 14, or 42?
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RegDwigнt♦Nov 29 '13 at 12:08

My descriptions here only serve to indicate the origin and relative age of the examples. The oldest verbs have the most inflected forms, in many cases (like went) because they have picked up parts of other verbs which are now at best archaic if not obsolete (wend).

Many of these forms are used with auxiliary verbs which may themselves be inflected. I am discounting those; as WS2 notes in his answer, calling those an “inflected form” yields more than thirteen anyway.

However, even if I have missed a few in each case, to get to thirteen will be very difficult and I would be interested in how your colleague identified that many.

It depends on what you include as 'inflectional'. If you mean compound tenses then I suppose you could get close to that number. Let's try:

I eat, I am eating, I was eating, I ate, I have eaten, I had eaten, I will eat, I will have eaten, I could eat, I could have eaten, I would eat, I would have eaten, to eat (infinitive).

Well that's 13 but I can think of others e.g. having eaten, having been eating, was eaten etc.

There are also the different person endings such as he eats, thou eatest, etc, as well as the person differences in the compound tenses with 'were' replacing 'was' and 'has' replacing 'have' in some of them.

‘Having ate’? You mean using the simple past as the past participle as in some dialects?
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Janus Bahs JacquetNov 28 '13 at 10:17

@JanusBahsJacquet If you are going to get into dialects there is no telling how many, or how few inflections you will get. In Norfolk, for example, 'eat' can be used as the past tense. 'I eat a whole apple pie yesterday'.
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WS2Nov 28 '13 at 11:34